1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus for forming an image on a paper sheet, OHP (OverHead Projector) film, intermediate image transfer body or similar recording medium and more particularly to a device included in the image forming apparatus for conveying the recording medium.
2. Description of the Background Art
Generally, in a recording medium conveying device included in an image forming apparatus, a sheet or similar recording medium is fed from a sheet cassette and conveyed along a preselected path. A roller pair nips the sheet fed from the sheet cassette and conveys it along the above path toward a registration roller pair. The registration roller pair once stops the sheet and then conveys it toward an image transfer station in synchronism with the rotation of an image carrier carrying a toner image thereon. At the image transfer station, the toner image is transferred from the image carrier to the sheet.
A problem with the conventional recording medium conveying device is that when, e.g., sheets are stacked on the sheet cassette in an inclined position, the sheet fed from the sheet cassette is apt to skew. It is a common practice to correct the skew of the sheet by causing the sheet to abut against the registration roller pair 2 and slacken.
Today, considering the increasing demand for the compact configuration of an image forming apparatus, it is difficult to provide a distance great enough for the sheet to sufficiently slacken on a path between the roller pair and the registration roller pair. However, if the distance between the roller pair and the registration roller pair and therefore the space available for the sheet to slacken is reduced, then heavy stress acts on the sheet and shifts it in, e.g., the widthwise direction. This brings about color shift in a color mode or brings about the bend of a straight line in a monochrome mode.
Japanese Paten Laid-Open Publication Nos. 7-261572 and 7-261573 each disclose a recording medium conveying device in which a pickup roller pays out sheets from a sheet cassette while a feed roller and a reverse roller cooperate to separate the top sheet from the underlying sheets. The sheet thus fed from the sheet cassette is conveyed by consecutive roller pairs until the leading edge of the sheet abuts against a registration roller pair, which is in a halt. Subsequently, the registration roller pair is rotated in synchronism with the rotation of a photoconductive drum carrying a toner image thereon, conveying the sheet. At this instant, an adhesion roller and an adhesion charger, which is disposed in the image transfer drum, cooperate to wrap the sheet around the drum being rotated. At a position short of an image transfer position, nip canceling means associated with the registration roller pair and roller pairs for conveyance is operated to cancel the nip acting on the sheet. The sheet is then brought to the image transfer position by the further rotation of the image transfer drum.
As stated above, the nip canceling means is associated with the registration roller pair, which conveys the sheet toward the image transfer position at preselected or reference timing, and cancels the nip of the roller pair. This, however, brings about a problem that the reference timing varies every time the nip of the registration roller is canceled, preventing the toner image from being transferred to the sheet with stable quality. Further, the charger and roller for adhesion increase the number of parts and overall size of the conveying device.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-91637.